1. Technical Field
The present invention relates in general to insulating polymer coatings on conductive metal surfaces, and in particular to polymer-cured zinc or tin-electroplate steel sheets. Still more particularly, the present invention relates to providing electrically nonconductive and corrosion resistant metal support surfaces in computer applications wherein the insulating surface can be breached by soldering or screws to create a conductive path between various computer components.
2. Description of the Related Art
Coating electrically conductive metal surfaces with polymeric materials is of interest in various electronics applications. The benefits of such polymer coatings are typically two-fold: protection of the metal surface from corrosion and to change the electrical properties of the substrate surface. The art in this area is diverse, with inventions distinguished by variations ranging from the identity of the substrate-metal surface on which the polymer is applied to the identity of the polymer. Also, various inventions have focused on the method of coating the metal surface with the polymer coating.
Palisin (U.S. Pat. Nos. 3,961,993 and 3,977,839) discloses a method of coating metal surfaces for corrosion protection, specifically, zinc, cadmium, nickel, or chromium-plated surfaces. The method in Palisin involves heat curing multiple layers of polymers onto the metal surface. Savelkouls (U.S. Pat. No. 4,101,356) discloses a method of polymer coating sheet steel by directing radio frequency radiation through a polymer stratum onto the steel surface. More complex applications followed in the Bolon et al. (U.S. Pat. No. 4,388,371) invention wherein several layers of various conductive and insulating polymer materials are heat cured onto an electrically conductive substrate.
Some more recent inventions include acrylic polymer coatings on steel articles that exhibit high corrosion resistance and low hydrogen reembrittlement characteristics (Hsu et al., U.S. Pat. No. 4,529,487); using conductive polyanaline coating on steel, iron, or aluminum substrates to impart corrosion resistance upon the substrate (Wrobleski et al., U.S. Pat. No. 5,658,649); and application of conductive polyaniline to metal substrates through evaporation of a solution of the polymer in contact with the metal surface (MacDiarmid et al., U.S. Pat. No. 5,645,890). More recently, van Ooij et al. (U.S. Pat. No. 5,759,629) have disclosed a method of imparting corrosion resistance to steel and aluminum sheets by paint-coating hydrolyzed vinyl silane to the surface, followed by application of other polymers.
Yoshimi et al. (U.S. Pat. No. 5,795,660) have invented a method of imparting corrosion resistance to chromate film-coated zinc alloy metal sheets by application of an organic resin on the metal substrate in conjunction with the chromate film. The resin film has a thickness of between 0.2 and 2.0 micrometers and contains various functional groups. The application is directed primarily to substrates exposed to highly corrosive environments, especially automobile parts exposed to salted road surfaces.
Some problems involving the application of metal surfaces in electronic components remain unsolved. In particular, the prevention of zinc or tin whisker growth on zinc or tin-plated sheet steel substrates is of great importance. The use of tin-plated sheet steel has grown recently due to the lower cost of materials over stainless steel. Generally, this tin-plated sheet steel is 0.008 to 0.010 inches in thickness, has a so called No. 25 or No. 50 tin coating weight on both sides, and is available world wide for use as EMC liners or similar computer chassis parts. However, tin whisker growth limits the practical use of this substrate for its desired electronics applications. Tin whiskers are hairlike crystals which grow spontaneously from the surface of the tin sheet steel. The growth of tin whiskers may result in flash-over, short circuits, and audible noise in electronic applications. Thus, there is a need to avoid this problem while exploiting the advantages of tin-plated sheet steel.
Polymer coatings can greatly enhance the usefulness of tin-plated sheet steel. By roll coating and heat curing a thin organic layer onto the tin plated steel, a significant corrosion resistance improvement can be achieved, while at the same time decreasing the tin whisker problem. Additionally, by using a specific type of organic coat and controlling the cure temperature, the zinc or tin-plated sheet steel can be stress relieved to help eliminate the growth of tin whiskers.
Although organic coated zinc or tin plated steel is already available, it is not available in the desired organic coating thickness. The thickness of the coating is critical in electronics applications because there is a desire to balance the conflicting need for imparting corrosion resistance on the one hand, while allowing for electrical connections to be made on the metal substrate on the other. If the polymer layer is too thick, it is difficult to solder through and dissipate the resin, thus making an electrical connection. If the polymer layer is too thin, corrosion resistance is diminished and tin whisker growth increases.
It would be desirable, therefore, to provide a method for an improved alloy plated sheet steel cured with a thin layer of insulating polymer material. The material would be characterized as a nonconductive breachable metal substrate due to its insulative properties and ability to easily breech the insulative layer to form electrically conductive pathways. Such a material would be ideal for use in products ranging from desktop PC's to high end server products and also other applications in the automotive industry.